The present invention relates to a secondary pressurization casting method, in which molten metal is injected into and filled in a cavity of a mold, and a secondary pressurizing force is exerted upon the filled molten metal by forwardly moving a secondary pressurization pin penetrating through the wall of the mold.
A die casting method in which molten metal is poured into a cavity of a mold at a high pressure, has been widely employed and practiced as a method most suitable for mass-production in the casting technique for aluminum alloys. In this die casting method, blow holes are liable to be produced in a thick wall portion of a product, and there is a tendency that a crystalline structure would become coarse and a mechanical strength of the thick wall portion would be lowered.
In order to resolve this problem, a secondary pressurization casting method, in which an additional pressurizing force (a secondary pressurizing force) is applied to molten metal injected into and filled in a cavity of a mold, was proposed (for example, see Japanese Patent Publication No. 48-7570 (1973) and Japanese Patent Publication No. 49-36093 (1974)).
As the method for carrying out the secondary pressurization, a method of pushing a secondary pressurization pin into molten metal after completion of filling of molten metal, and a method of forwardly moving a secondary pressurization pin within a through-hole that is formed in a mold for the purpose of making the secondary pressurization pin advance or retreat and thereby pushing back the molten metal entering into the through-hole, are known.
In the die casting method, during the period when molten metal is being injected into and filled in a cavity, a pressure of several ten kgf/cm.sup.2 is exerted upon the cavity wall surface, and upon completion of filling, this pressure would reach even a pressure of several hundreds kgf/cm.sup.2. This molten metal pressurized to such a high pressure would enter even into minute gaps in the mold, and would form casting fins. Therefore, a casting fin is produced at the periphery of the tip end of a secondary pressurization pin inserted into a through-hole that is formed in a mold with a minute gap retained therearound. This casting fin would prevent the movement of the secondary pressurization pin due to a wedge effect, and in the worst case the secondary pressurization pin would be locked and would become unable to move.